1. Field of the Invention
The present invention relates generally to the field of coal classifiers and pulverizers used to crush and pulverize and sort furnace fuel coal for combustion, and in particular to a new and useful dynamic classifier having a hollow shaft drive motor for the top section of a vertical axis coal pulverizer.
2. Description of the Related Art
The use of dynamic, or externally driven, coal classifiers to improve control of product particle size distribution is coming into increasing prominence as a supplemental technology to low NO.sub.x coal burners for reducing environmental pollutants. Dynamic coal classifiers are currently used to enhance the control of coal particle size and to improve the capacities of vertical-spindle pulverizers. A large percentage of dynamic coal classifiers are retrofit on existing pulverizers. High costs and difficult installation are the biggest barriers to widespread retrofitting of dynamic classifiers to existing pulverizers.
Vertical-spindle pulverizers usually receive raw coal through a feed pipe at the center of the grinding table rotation. The classifier is concentric with and rotates above and about the same grinding table center axis. Further, the coal output lines to the burners are usually positioned around the top of the pulverizer. The output lines are usually four or more pipes extending from the top of the pulverizer. This orientation of the input and output pipes on the pulverizer gives rise to special problems with respect to arranging the classifier's mechanical drive components.
Since the central axis of the classifier is occupied by the feed shaft, vertically-oriented motors used to drive the classifier rotation must be connected to the classifier rotor by V-belt or chain drives positioned to one side of the feed pipe. Alternatively, the motor may be connected to the classifier by gear-type speed reducers having large, hollow shafts. In each case, potential problems arise with respect to interference of drive components with planned or existing output pipes.
Speed reduction or control from the drive shaft of electric motors typically used to operate a classifier is required to obtain the low and variable rotation speeds desired. An electric or hydraulic motor in combination with speed reduction mechanisms is commonly used with classifiers to obtain operating rotational speeds of between 50 and 200 rpm.
These known classifiers and drive mechanisms have several drawbacks, including the high cost and difficult retrofit installation. The additional space occupied by the classifier drive mechanism is premium and sometimes difficult to obtain, such as when the height of the pulverizer must be increased as a result of the retrofit of the dynamic classifier.